The present invention relates to a plastic pipe having a multi-layer casing which includes an elastic barrier layer which is virtually impermeable to oxygen and to a process for the production of the pipe.
The plastic pipe, encased in accordance with the invention, is used particularly in the heating industry for underfloor hot water heating systems.
In addition to steel or copper pipes, plastic pipes, in particular pipes made of polyolefins, have, in recent years, become important in the construction of heating systems.
However, plastic pipes have the disadvantage that their walls have an undesirably high permeability to oxygen, even at room temperature. A pipe composed, for example, of high density polyethylene and having a wall thickness of, for example, 2 mm and an external diameter of 20 mm, has a permeability to oxygen of approximately 3 cm.sup.3 /m of pipe length per day and per bar at 23.degree. C.
Oxygen, therefore, enters continuously into the heating circulation via the hot water circulating as heat transfer agent in the pipes of the heating system. The oxygen thus comes into contact with iron components of the heating system, for example, the boiler or the radiators. The iron components present in the heating circuit are, therefore, subject to corrosion, and there is a risk that they may become unserviceable after some years of use as a result of this corrosion.
In order to eliminate this disadvantage of plastic pipes, it has been suggested that the plastic pipes used in heating systems should be surrounded with a casing composed of a film laminate comprising a layer of stretch-oriented polyester film as a supporting material, an aluminum foil as a barrier layer impermeable to oxygen and a layer of hot-melt adhesive.
The polyester film forms the outer side of the casing. The aluminum foil layer is adjacent to the surface of the pipe and is joined to the latter by an adhesive bond.
However, the proposed casing sheath has the disadvantage that its aluminum layer is no longer adequately impermeable to oxygen after it has been subjected to stretching when the pipe is bent and also as a result of thermal expansion of the pipe, since these factors cause it to crack. The thermal expansion of the pipe has the effect that the coefficient of thermal expansion of the plastic forming the pipe is considerably greater than the coefficient of the thermal expansion of the aluminum barrier layer which is attached to the pipe.
In some cases, the encased pipes are bent in order to convert them into a curved state. The casing surrounding the pipe is subjected to considerable strain through bending the pipe, particularly if the bent pipe elbow has a small radius.
After the encased pipes have been installed, the barrier layer of the casing is subjected to strain because the diameter of the pipe increases when the pipe is carrying hot water.
In a section of the aluminum foil barrier layer which is subjected to strain, this layer is no longer impermeable to oxygen.